It is oftentimes necessary, or at least desirable, that various electrical instruments be monitored and an indication established if a fault should occur at any time in the instrument such that the instrument might not thereafter be suitable for its intended purpose. Such is the case, for example, where integrating meters, such as the common watthour meter, is concerned. As is well known, the induction watthour meter normally includes a rotatable disk the speed of which is made proportional to the power delivered to the metered load. Hence, the total number of revolutions by the disk is proportional to the energy consumed so that when used with a calibrated meter, the meter will cumulatively indicate the amount of electrical energy consumed by the load connected with the meter. In the watthour meter, damping, or drag, magnets are commonly utilized to retard disk speed of rotation to thus cause the speed to be made proportional to the power flow through the meter. It is important that the strength of the damping magnets be constant when utilized in the meter since any change in strength after the meter is calibrated and put into service will cause an error in meter readings.
It has heretofore been common when utilizing damping magnets for watthour meters to partially de-magnetize permanent magnets before using the same in commercial meters. Such a process has been successful in stablizing such magnets for use in watthour meters that are of sufficient reliability so as not to adversely affect watthour meter readings in normal use. It has been found that under unusually severe power frequency faults, the meter may experience a heat producing fault current of sufficient magnitude to adversely affect the permanent magnets causing errors in meter readings. Since most possibly damaging current surges produce heat in the current coils, monitoring of these coils by a heat sensitive fault indicator enables continual meter monitoring and indications to be made if a possibly damaging surge occurs. Obviously, if a possibly damaging current surge has occurred, then it is important that the then possibly unreliable meter be tested to insure continued reliability or be replaced, if necessary.
While devices for monitoring meters to indicate a surge have heretofore been known and/or utilized, none of these devices have been completely successful in solving the problem in a reliable manner and/or have not solved the problem in such a manner so as to make the indication of a received current surge readily discernible at the meter. Some such prior art devices have, for example, utilized various indicators such as, for example, indicating flags, fuses, or temperature sensitive bands or paints attached to the permanent magnet and caused to change color or fall off when an induced current surge is experienced. An example of this type of indicator is shown in U.S. Pat. No. 2,236,277 issued to G. R. Sturtevant on Mar. 25, 1941. The present invention is not frequency limited (such as the Sturtevant device).
In addition, an application of even date has been filed by W. C. Kemp and John M. Carr entitled "HEAT SENSITIVE FAULT DETECTING AND INDICATING DEVICE AND METHOD", which application is assigned to the assignee of this invention. The fault detecting and indicating device and method shown in this application are directed to the same end, the device and method are distinct from one another.